1. Field of the Invention
The present invention relates generally to wireless communication systems and, in particular, to call admission control in wireless communication systems.
2. Description of the Related Art
Wireless communication systems employ Code Division Multiple Access (xe2x80x9cCDMAxe2x80x9d) modulation techniques to permit a large number of system users to communicate with one another. Such systems work because each signal is coded with spreading sequences, such as with pseudo-random noise (xe2x80x9cPNxe2x80x9d) sequences, and orthogonal spreading sequences, such as Walsh codes. This coding permits signal separation and signal reconstruction at the receiver. In typical CDMA systems, communication is achieved by using a different spreading sequence for each channel. This permits a plurality of transmitted signals to share the same bandwidth. Particular transmitted signals are retrieved from the communication channel by despreading a signal from all of the signals. Despreading is achieved by using a known user despreading sequence related to the spreading sequence implemented at the transmitter.
FIG. 1 illustrates CDMA system 10. The geographic area serviced by CDMA system 10 is divided into a plurality of spatially distinct areas called xe2x80x9ccells.xe2x80x9d Although cells 2, 4, 6 are illustrated as a hexagon in a honeycomb pattern, each cell is actually of an irregular shape that depends on the topography of the terrain surrounding the cell. Each cell 2, 4, 6 contains one base station 12, 14, and 16, respectively. Each base station 12, 14, and 16 includes equipment to communicate with Mobile Switching Center (xe2x80x9cMSCxe2x80x9d) 20, which is connected to local and/or long-distance transmission network 22, such as a public switch telephone network (PSTN). Each base station 12, 14, and 16 also includes radios and antennas that the base station uses to communicate with mobile terminals 24, 26.
When a call is set up in a CDMA system, a base station and mobile terminal communicate over a forward link and a reverse link. The forward link includes communication channels for transmitting signals from the base station to the mobile terminal, and the reverse link includes communication channels for transmitting signals from the mobile terminal to the base station. The base station transmits certain types of control information to the mobile terminal over a communication channel, referred to herein as a forward control channel, also known in the art as a forward overhead channel. Forward control channels include the pilot, paging, and synchronization channels. The base station transmits voice or data, and certain types of control information over a communication channel, referred to herein as a forward traffic channel. The signals on the communication channels are organized in time periods, referred to herein as frames. (Frames are typically 20-millisecond (ms) in length. The signals transmitted over the control channels are referred to herein as control signals, and the signals transmitted over the traffic channels are referred to herein as traffic signals.
Some base stations may implement overload control to ensure that the power transmitted by the base station does not exceed the power level at which the base station""s equipment is designed to operate over an extended time period. The base stations implement overload control by using one of several remedies. Typically, one of these remedies includes denying access to any new call requests, referred to herein as call blocking. Typically, call blocking is initiated when the total power level of all the signals transmitted by the base station reaches a certain percentage, for example, 80% to 100% of the maximum output power level at which the base station""s amplifier is designed to operate over an extended time period. This maximum output power is referred to herein as the amplifier""s maximum-continuous power level.
The inventors have discovered that problems can result from initiating call blocking based on only the total power level of the signals transmitted by the base station. Generally, when a call is added to a cell, the noise level in the cell and in the surrounding cells is increased. This makes it more difficult for a mobile terminal to clearly obtain the pilot, particularly if the mobile terminal is at the edge of a cell. In some base stations the pilot""s power level can vary. In the case where the pilot""s power level is reduced, it is even more difficult for a mobile terminal at the edge of a cell to obtain the pilot. When the mobile terminal cannot obtain a clear and continuous pilot, problems can result on the call between this mobile terminal and the base station. These problems can range from not being able to despread a frame, which results in an erred frame, to dropping a call, which results in an inconvenience to the customer and a loss of revenue. Therefore, although the total power level is still below the percentage at which calls are blocked, adding the call may cause calls already in the cell to have difficulties.
The invention solves the above problems by initiating call blocking responsive to a call-quality measurement of the forward link. A call-quality measurement is a measurement of how well a mobile terminal is able to receive the forward link.
One call-quality measurement is a pilot fraction of the forward link, which is a ratio of the pilot""s power level to the power level of a set of forward-link signals of a base station. Preferably, call blocking is initiated when an average pilot fraction is below a pilot-fraction blocking threshold. Alternatively, call blocking can be initiated when the pilot fraction is below the pilot-fraction blocking threshold.
The pilot""s power level is obtained for a time period, and the signal set""s power level is obtained for the same time period. The pilot fraction is determined for the time period. The pilot fraction is then used to determine the average pilot fraction for the current time period. The average pilot fraction for the current time period is based on the pilot fraction for the current time period and an average pilot fraction for a previous time period. Alternatively, the average pilot fraction for the current time period can be based on the pilot fraction for the current time period, and the pilot fractions of the previous plurality of time periods, averaged over the plurality of time periods.
When the average pilot fraction is below the pilot-fraction blocking threshold, call blocking is initiated. The pilot-fraction blocking threshold is preferably based on several factors. The first is the pilot fraction when the base station is at full load. The second is the size, shape, and terrain of the cell. The third is the aggressiveness of the overload control, which is the balance between the desire to increase the capacity of the system and the desire to ensure the signal is received at a particular quality. In the preferred embodiment, the signal set includes all of the signals generated by the base station, alternatively, the signal set can include fewer than all the signals generated by the base station. For example, the signal set can include a plurality of traffic signals, or a plurality of traffic signals and one or more of the control signal.
If the cell includes several sectors, the call blocking is initiated on a sector basis. Call blocking is initiated when the average pilot fraction of the sector is below the pilot-fraction blocking threshold.
Other call-quality measurements, such as the forward link""s frame error rate, the number of dropped calls, or the Power Measurement Report Message (PMRM), can be used, alone or in combination, instead of or in addition to the pilot fraction of the forward link to determine whether call blocking should be initiated. Determining whether call blocking should be initiated using one of the other call-quality measurements is performed in a similar manner as described above for the pilot fraction. Determining whether call blocking should be initiated using several call-quality measurements involves determining whether call blocking should be initiated based on any of the call quality measurements and initiating call blocking when any one of the call-quality measurements indicates that call blocking should be initiated. Alternatively, call blocking can be initiated when several of the call quality measurements indicate that call blocking should be initiated.